1. Field of the Invention
The present invention relates to an addition curable silicone resin composition and a silicone lens that uses the composition, and relates particularly to an addition curable silicone resin composition that yields a cured product with minimal surface tack and favorable strength properties, and a silicone lens that is formed using this cured product and exhibits favorable levels of transparency and moldability, excellent shock resistance, and almost no surface tack.
2. Description of the Prior Art
Addition curable silicone rubber compositions form cured products that exhibit excellent properties of weather resistance and heat resistance and the like, as well as superior rubber-like properties such as hardness and elongation, and they are consequently used as coating materials and the like in a wide variety of applications (see patent reference 1). However, because the resulting cured products also exhibit surface tack, dust adhesion becomes a problem when these compositions are used as coating agents or the like for electrical or electronic components.
Cured products comprising hard resins obtained from silicone varnishes exhibit minimal surface tack, meaning the problem of dust adhesion does not arise, but such cured products suffer from a different problem in that because they lack adequate shock resistance, they tend to be prone to cracking, and particularly to cracking caused by thermal shocks.
It is known that adding a resin-like organopolysiloxane component to an addition-curable silicone rubber composition improves the strength of the cured product. For example, the patent reference 2 discloses an improvement in the strength of the cured product by using a branched (in other words, resin-like) organopolysiloxane as the alkenyl group-containing organopolysiloxane that is subjected to an addition reaction with an organohydrogenpolysiloxane. However, even in those cases where the strength of the cured product is increased by using a resin-like organopolysiloxane, surface tack and dust adhesion remain problems.
Accordingly, in the field of electrical and electronic component packaging, there is considerable demand for a silicone composition capable of forming a cured product that does not suffer from dust adhesion to the cured product surface, and exhibits excellent levels of crack resistance and shock resistance. In chip coating and casting applications, the use of solid or semisolid materials is problematic, and consequently a composition that is liquid and yet yields a tough cured product upon curing has been keenly sought.
The lenses for LED light emitting devices are conventionally formed by using a mechanical molding technique such as injection molding to mold a thermoplastic resin such as an acrylic resin or polycarbonate resin. However, as the output of such LED light emitting devices increases, these thermoplastic resins become less satisfactory in terms of their heat resistance and resistance to discoloration. Furthermore, because lead-free solders, which have recently become widespread, have a higher melting temperature than conventional solders, optical elements are typically soldered to substrates at temperatures of 260° C. or higher. If soldering is conducted at this type of high temperature, then lenses formed using conventional thermoplastic resins not only undergo deformation, but also suffer from yellowing as a result of the high temperature. As a result of these circumstances, a multitude of tests have been conducted into producing the lenses for LEDs and the like using silicone resins (for example, see patent reference 3).
However, when an addition curable silicone resin composition is used, the problems described above are evident. Namely, if a lens with a hollow structure is molded using the type of silicone material used in conventional lenses, then a hard silicone resin tends to yield a lens with a low degree of shock resistance, whereas a soft silicone rubber is prone to deformation, meaning that not only is the reliability of the lens optical properties poor, but the LED chip or wires or the like encapsulated within the rubber tend to be placed under stress. Moreover, with a low-viscosity silicone resin, burr generation also tends to be a problem during molding.
[Patent Reference 1] US 2004/0214966 A1
[Patent Reference 2] JP 2005-76003 A
[Patent Reference 3] JP 2006-324596 A